This invention relates to the clipping of digital signals, and more particularly to clipping of digital video luminance and chrominance components so that the resulting composite digital video signals are within desired clipping limits.
A typical prior art digital clipping circuit is shown in FIG. 1. As long as the value of the input signal is less than the positive clipping level and greater than the negative clipping level, the input signal is passed through the multiplexers to be the clipped signal output. However, if the value of the input signal becomes greater than the positive clipping level, the positive clipping level is substituted for the input signal as the clipped signal output. And, similarly, if the value of the input signal becomes less than the negative clipping level, the negative clipping level is substituted for the input signal as the clipped signal output.
Ideally, performing clipping operations on a signal input such as that shown in FIG. 2A leads to a clipped signal output as shown in FIG. 2B. However, in a digital implementation of a clipping circuit, as the frequency of the signal being clipped increases and begins to approach half of the sampling frequency, clipping of the individual sampled points does not necessarily provide adequate clipping of the reconstructed analog signal.
FIGS. 3A through 3C illustrate the problem that results from a low sampling rate relative to the frequency of the signal being clipped. The waveform shown in FIG. 3A clearly exceeds the positive and negative clipping levels indicated by the dotted lines. Even though four samples are taken, only one of these is a sample that occurs during the time that the signal is in excess of the clipping levels, and that sample occurred when the signal was only slightly above the positive clipping threshold. The whole negative excursion of the waveform below the negative clipping threshold occurred between the third and fourth sample points and, consequently, was not detected and produces no clipping. Thus, it can be seen that, at a low relative sampling rate, the effectiveness of the clipping of the signal depends on the phase relationship between the signal and the sampling times.
As can be seen in FIGS. 3B and 3C, even though the second sample is clipped to the positive clipping level and this somewhat diminishes the positive going amplitude of the reconstructed waveform, the reconstructed waveform still exceeds both clipping levels, especially the negative one. Moreover, the partial clipping that does occur introduces into the reconstructed waveform phase distortion and discontinuities that appear as higher frequency components.
Of course, a higher sampling rate is always one approach to avoiding this problem, but oversampling itself can produce some undesirable effects such as ringing, and is not economically feasible or technically desirable in most circumstances.
As shown in FIG. 7, conventional video processing amplifiers of the prior art generally include a circuit to permit controllable clipping of the luminance level before it is re-combined with the chrominance signal to produce the composite video and another circuit to permit controllable clipping of the composite video signal after the luminance and chrominance have been combined. A limitation that is inherent in this approach is that the composite signal is "hard" clipped without any attention to the subtleties of its component parts.